1. Field of the Invention
The present invention relates to a surface mount antenna capable of transmitting and receiving signals (radio waves) in different frequency bands and also to a communication device such as a portable telephone including such an antenna.
2. Description of the Related Art
In recent years, it is needed to commercially provide a single terminal having a multi-band capability for use in plural applications such as GSM (Global System for Mobile communication systems), DCS (Digital Cellular System), PDC (Personal Digital Cellular telecommunication system), and PHS (Personal Handyphone System). To meet the above requirement, Japanese Unexamined Patent Application Publication No. 11-214917 discloses a multiple frequency antenna of the surface mount type capable of transmitting and receiving signals in different frequency bands.
In this antenna, as shown in FIG. 22A, a dielectric member 105 is disposed on a ground plate 101, and a conductive plate 102 having a cut-out 106 is disposed on the upper surface of the dielectric member 105. When a signal is supplied via a feeding line 104, a current in a fundamental mode flows through the conductive plate 102, along a path L1 from the side of a short-circuiting plate 103 toward the opposite side, and a current in a high-order mode (third-order mode in this specific example) flows along a path L3. Thus, this antenna has a frequency characteristic such as that shown in FIG. 22B and is capable of transmitting and receiving signals at two different frequencies: a resonance frequency f1 in the fundamental mode; and a resonance frequency f3 in the high-order mode.
Note that in the present description, the fundamental mode refers to a resonance mode having the lowest resonance frequency of those in various resonance modes, and the high-order modes refer to resonance modes having resonance frequencies higher than the resonance frequency in the fundamental mode. When it is necessary to distinguish the respective high-order modes from each other, they are denoted by a second-order mode, a third-order mode, and so on in the order of increasing resonance frequencies.
In the case where currents in the fundamental mode and a high-order mode are passed through the same conductive plate 102 from its one end to the opposite end as in the conventional antenna described above, the difference between the resonance frequencies in the respective modes is determined by the difference between the lengths of the current paths. In general, the distance from one end to the opposite end of the conductive plate 102 is determined on the basis of the fundamental mode such that it becomes substantially equal to one-quarter the effective wavelength 1 in the fundamental mode (in other words, the resonance frequency in the fundamental mode is determined by the above-described distance). In order to set the resonance frequency in a high-order mode to a desired value, it is required that the length of the current path in the high-order mode should be different by a corresponding amount from the length of the current path in the fundamental mode. In the conventional technique described above, a difference in current path length is created by forming the cut-out 106 at a location where the current in the high-order mode becomes maximum thereby changing the current path L3 in the high-order mode so as to have a greater length required to set the resonance frequency f3 in the high-order mode to the desired value.
In the conventional technique described above, because the same conductive plate 102 is used for resonance in both the fundamental mode and the high-order mode, the size of the antenna can be reduced compared with the size of an antenna in which resonance in the fundamental mode and resonance in the high-order mode are achieved using different conductive plates. However, in the conventional technique described above, it is required that the cut-out 106 should be formed in the conductive plate 102, and thus the conductive plate 102 should be large enough to form the cut-out 106. This makes it difficult to achieve a further reduction in the size of the antenna.
Furthermore, in the conventional technique described above, the current path in the high-order mode is curved by the cut-out 106 thereby increasing the length thereof. Therefore, the change in the length of the current path is limited within a small range determined by the change in the perimeter of the cut-out 106 (that is, the change in the shape of the cut-out 106). Thus, it is difficult to set the difference between the resonance frequency in the fundamental mode and the resonance frequency in the high-order mode over a large range.
Furthermore, it is difficult to precisely control the resonance frequency in the high-order mode by adjusting the perimeter (shape) of the cut-out 106, and thus it is difficult to efficiently produce and provide an antenna having high performance and high reliability.
In view of the above, it is an object of the present invention to efficiently and economically provide a high-performance, high-reliability, small-sized surface mount antenna having features that the difference between the resonance frequencies in the fundamental mode and the high-order mode can be adjusted and set over a wide range, and both the resonance frequencies in the fundamental mode and the high-order mode can be precisely set to desired values, and also to provide a communication device including such an excellent antenna.
According to an aspect of the present invention, to achieve the above object, there is provided a surface mount antenna comprising: a dielectric substrate; and a radiating electrode formed on the dielectric substrate, one end of the radiating electrode being an open end, a feeding electrode or a ground terminal being formed on the opposite end of the radiating electrode, wherein the radiating electrode includes a first part having a small electrical length per unit physical length and a second part having a greater electrical length than the small electrical length, the first part and the second part being arranged in series along a current path between the one end and the opposite end.
According to another aspect of the present invention, there is provided a surface mount antenna comprising: a dielectric substrate; and a radiating electrode formed on the dielectric substrate, one end of the radiating electrode being an open end, a feeding electrode or a ground terminal being formed on the opposite end of the radiating electrode, wherein the radiating electrode includes a first part in which a resonance current in a fundamental mode becomes maximum and a second part in which a resonance current in a high-order mode becomes maximum, the first part and the second part being arranged in series along a current path between the one end and the opposite end; and at least one of the first and second parts includes an inductance component disposed in series in the current path.
Preferably, the inductance component is formed by a meander electrode pattern.
Alternatively, the inductance component may be formed by a capacitance component connected in parallel to the first part or the second part.
The radiating electrode may be formed by a helical electrode pattern, and the inductance component may be formed by reducing the distance between adjacent electrodes of the helical electrode pattern.
The inductance component may also be formed by a member having a high dielectric constant, the member being disposed in the first part or the second part.
The surface mount antenna may further comprise a non-feeding radiation electrode formed adjacent the radiating electrode, the resonance mode associated with the non-feeding radiation electrode forms multiple resonance in conjunction with at least one of the fundamental mode and the high-order mode associated with the externally-connected electrode.
The non-feeding radiation electrode may include a part having a small electrical length per unit physical length and a part having a greater electrical length than the small electrical length, the parts being arranged in series along a path of a current flowing through the non-feeding radiation electrode.
The non-feeding radiation electrode may include a first part in which a resonance current in a fundamental mode becomes maximum and a second part in which a resonance current in a high-order mode becomes maximum, the first part and the second part being arranged in series along a path of a current flowing through the non-feeding radiation electrode, and at least one of the first and second parts may include an inductance component disposed in series in the current path.
The inductance component may be formed by a meander electrode pattern.
Alternatively, the inductance component may be formed by a capacitance component connected in parallel to the first part or the second part.
The radiating electrode may be formed by a helical electrode pattern, and the inductance component may be formed by reducing the distance between adjacent electrodes of the helical electrode pattern.
The inductance component may also be formed by a member having a high dielectric constant, the member being disposed in the first part or the second part.
Preferably, the vector direction of a current flowing though the radiating electrode and the vector direction of a current flowing though the non-feeding radiation electrode are perpendicular to each other.
According to another aspect of the present invention, there is provided a communication device including one of the surface mount antennas described above.
In the present invention, for example, a meander pattern is formed in one of or both of maximum resonance current parts in the fundamental mode and the high-order mode in the current path of the feeding radiation electrode so that a series inductance component is locally added therein thereby making the electrical length per unit physical length therein become greater than in the other parts. Thus, the feeding radiation electrode includes a series of parts which are arranged such that the electrical length per unit physical length is alternately large and small from one part to another.
As described above, it is possible to vary the difference between the resonance frequency in the fundamental mode and the resonance frequency in the high-order mode by locally adding the series inductance component in one of or both of the maximum resonance current part in the fundamental mode and the maximum resonance current part in the high-order mode thereby increasing the electrical length therein. Furthermore, by locally changing the value of the series inductance component, it is possible to easily change the resonance frequency in the mode associated with the series inductance component added in the maximum resonance current parts, independently of the other mode. Besides, the change or adjustment of the resonance frequency by means of changing the series inductance component can be performed over a large range. Therefore, it is possible to adjust or set the difference between the resonance frequency in the fundamental mode and the resonance frequency in the high-order mode over a large range. This makes it possible to easily and efficiently provide a surface mount antenna having a frequency characteristic satisfying requirements needed in a terminal for use in multi-band applications. Furthermore, the degree of freedom for the design of the antenna is improved. Besides, a reduction in cost of the surface mount antenna can be achieved, and the performance and the reliability of the surface mount antenna can be improved.
The meander pattern or the like used to add the series inductance component can be added without causing a significant increase in the area of the feeding radiation electrode, and thus it is possible to realize a surface mount antenna having a small size.